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From: Terry Lambert <terry@lambert.org>
Message-Id: <199701231831.LAA24098@phaeton.artisoft.com>
Subject: Re: CRC-16 algorithms?
To: msmith@atrad.adelaide.edu.au (Michael Smith)
Date: Thu, 23 Jan 1997 11:31:20 -0700 (MST)
Cc: hackers@FreeBSD.ORG
In-Reply-To: <199701231259.XAA21523@genesis.atrad.adelaide.edu.au> from "Michael Smith" at Jan 23, 97 11:29:27 pm
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> (Yes, this is FreeBSD-related 8)
> 
> I'm looking for any implementations of various CRC-16 polynomials; I'm
> trying to compute a checksum for a parameter save area for a
> peripheral, which is documented as "CRC-16", but no details of the
> polynomial are given. 8(
> 
> If you're into cracking these sorts of things, here's the parameter space :
> 
> 00000000  00 39 3c 3e 01 02 03 00  80 00 00 00 00 00 00 00  |.9<>............|
> 00000010  08 00 0e 20 3d 4b 00 00  00 00 00 00 00 02 01 00  |... =K..........|
> 00000020  01 03 00 de ad 01 00 00  00 00 00 00 00 00 00 01  |....-...........|
> 00000030  00 00 00 00 00 00 00 00  00 00 00 00 01 26 3c aa  |.............&<.|
> 
> The bytes at 0x3d and 0x3e are the CRC-16 of the bytes from 0-0x3c
> inclusive.  Any help here will help make the DOS configuration utility
> for this device unnecessary, which would be nice 8)

Brute-force it; there are only 65536 * 2 * 4^2 possibilities, and
of these, there are only 32768 "likely" values (non-reflected, polynomial
value ends in 1, W XOR value is all 0's).

Here is a pointer to code for generalized CRC algorithms:

	http://www.strangecreations.com/strange/library/misc/crc.txt

It also contains code for table generation for building a table-driven
implementation once you find the correct polynomial.


					Regards,
					Terry Lambert
					terry@lambert.org
---
Any opinions in this posting are my own and not those of my present
or previous employers.